DC-DC converter in a non-steady system

ABSTRACT

Multiphase electromagnetic machines, such as free-piston engines or compressors, may require, or supply, a pulsed power profile from or to a DC bus, respectively. The pulsed power profile may include relatively large fluctuations in instantaneous power. Sourcing, sinking, or otherwise exchanging power with an AC grid, via an inverter, may be accomplished by using an energy storage device and a DC-DC converter coupled to a DC bus. The energy storage device may aid in smoothing the pulsed power profile, while the DC-DC converter may aid in reducing fluctuations in voltage across a DC bus due to energy storage in the energy storage device.

The present disclosure is directed towards a DC-DC converter in anon-steady system, such as in a free-piston machine. More particularly,the present disclosure is directed towards a DC-DC converter being usedto manage DC power to or from a non-steady system, such as a free-pistonmachine. This application claims the benefit of U.S. Provisional PatentApplication No. 62/561,167 filed Sep. 20, 2017, the disclosure of whichis hereby incorporated by reference herein in its entirety.

BACKGROUND

Systems that convert between kinetic energy and electrical energysometimes exhibit large output voltage fluctuations, especially if nomechanical energy storage device is used. In a conventionalpiston-cylinder device, for example, a flywheel can maintain a nearlyconstant crankshaft speed, even though the extraction of energy isdiscontinuous by nature. For example, in the context of a conventionalpiston engine, energy is transferred to the crankshaft during anexpansion stroke, while it is removed from the crankshaft during acompression stroke. Conventional engines can therefore be connected to aconstant power load, such as an electric motor, without any additionaldevice to manage the energy transfer. Systems that do not include aflywheel or equivalent device, such as a free-piston machine, forexample, operate with discontinuous energy transfers.

Systems that are connected to an electrical grid must provide power thatmeets the requirements of the grid. Large fluctuations, on a DC buscoupled to a system, for example, place operational demands on acorresponding grid tie inverter (GTI). Additionally, operation of thesystem is constrained to limit fluctuations of voltage of the DC bus.

SUMMARY

In some embodiments, a power management system that receives a powerinput from a power generator includes a DC bus, at least one energystorage device, and a DC-DC converter coupled to the DC bus. The DC-DCconverter is configured to output a smoothed power output relative tothe power input. For example, in some embodiments, the power generatorincludes a free-piston engine. In a further example, in someembodiments, the DC bus includes two buses, having a relative voltagebetween each other. In a further example, in some embodiments, a powermanagement system includes an inverter, wherein the grid tie inverter iscoupled to the output.

In some embodiments, a power management system includes circuitrycoupled to the DC bus. The circuitry is configured to control aplurality of currents in a plurality of respective phases of the powergenerator and output a pulsed power profile to the DC bus. For example,the free-piston engine may include one or more linear electromagneticmachines which include a respective plurality of phases.

In some embodiments, the DC-DC converter is a first DC-DC converter, theinverter is coupled to at least one of a power grid and a load, and thesystem includes a second DC-DC converter coupled to the energy storagedevice and to the inverter.

In some embodiments, the at least one energy storage device includes atleast one capacitor. In some embodiments, the at least one energystorage device includes a battery system. In some embodiments, the atleast one energy storage device is configured to store and releaseelectrical energy from the DC bus.

In some embodiments, the output has a corresponding voltage, and theDC-DC converter is configured to regulate the voltage. In someembodiments, the DC-DC converter is adjustable, and the power managementsystem includes a control system configured to adjust one or moreoperating characteristics of the DC-DC converter (e.g., a voltage). Insome embodiments, the DC-DC converter includes a step-down converter, astep-up converter, or both.

In some embodiments, a power management system that receives power froma power input and provides a power output to a free-piston machineincludes a DC bus, at least one energy storage device, and a DC-DCconverter coupled to the DC bus. The DC-DC converter is configured tooutput a pulsed power output to the free-piston machine relative to thepower input. For example, in some embodiments, the power generatorincludes a free-piston engine. In a further example, in someembodiments, the DC bus includes two buses, having a relative voltagebetween each other. In a further example, in some embodiments, a powermanagement system includes a grid tie inverter, wherein the grid tieinverter is coupled to the input.

In some embodiments, the power management system includes circuitrycoupled to the DC bus. The circuitry is configured to control aplurality of currents in a plurality of respective phases of an electricmotor of the free-piston machine, and receive a pulsed power profilefrom the DC bus.

In some embodiments, the DC-DC converter is a first DC-DC converter, andthe system includes an inverter coupled to a power grid, and a secondDC-DC converter coupled to the energy storage device and to theinverter.

In some embodiments, the at least one energy storage device includes atleast one capacitor. In some embodiments, the at least one energystorage device includes a battery system. In some embodiments, the atleast one energy storage device is configured to store and releaseelectrical energy from the DC bus.

In some embodiments, the input has a corresponding voltage, and theDC-DC converter is configured to regulate the voltage. In someembodiments, the DC-DC converter is adjustable, and the power managementsystem includes a control system configured to adjust one or moreoperating characteristics of the DC-DC converter (e.g., a voltage). Insome embodiments, the DC-DC converter includes a step-down converter, astep-up converter, or both.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure, in accordance with one or more variousembodiments, is described in detail with reference to the followingfigures. The drawings are provided for purposes of illustration only andmerely depict typical or example embodiments. These drawings areprovided to facilitate an understanding of the concepts disclosed hereinand shall not be considered limiting of the breadth, scope, orapplicability of these concepts. It should be noted that for clarity andease of illustration these drawings are not necessarily made to scale.

FIG. 1 shows a diagram of an illustrative system, in accordance withsome embodiments of the present disclosure;

FIG. 2 shows a plot of illustrative power and voltage time traces, inaccordance with some embodiments of the present disclosure;

FIG. 3 shows a diagram of an illustrative system, including an energystorage device, in accordance with some embodiments of the presentdisclosure;

FIG. 4 shows a diagram of an illustrative system, including an energystorage device and a DC-DC converter, in accordance with someembodiments of the present disclosure;

FIG. 5 shows a diagram of an illustrative system, including an energystorage device and two DC-DC converters, in accordance with someembodiments of the present disclosure;

FIG. 6 shows a diagram of an illustrative system, including an energystorage device and integrated DC-DC converters, in accordance with someembodiments of the present disclosure; and

FIG. 7 shows a cross-section view of an illustrative free-pistonmachine, in accordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure is directed towards a DC-DC converter being usedto manage DC power to or from a non-steady system, such as a free-pistonmachine. The present disclosure provides architectures for providingpower interactions with a free-piston machine. The architectures mayinclude one or more DC-DC converters and energy storage devices, whileavoiding negative impacts on a grid tie inverter (GTI) and motor powerelectronics. In some embodiments, the present disclosure describesincluding one or more DC/DC converters on a DC bus to absorb the voltagefluctuations from an energy storage device.

As used herein, an energy storage device (ESD) shall refer to anysuitable device that is configured to store and release electricalenergy in quantities and on time scales of interest to the presentdisclosure (e.g., to pulsed power profiles). Illustrative examples of anESD include a battery or other electrochemical cell, a capacitor or bankof capacitors, an inductor, a magnetic field, or any combinationthereof.

As used herein, a DC-DC converter shall refer to any suitable devicethat is configured to convert electrical power between direct current(DC) voltage levels. A DC-DC converter may include, for example, anelectronic circuit, a magnetic device, an electromechanical device, anyother suitable device, or any combination thereof, having any suitabletopology.

FIG. 1 shows a diagram of an illustrative system 100, in accordance withsome embodiments of the present disclosure. System 100 includesfree-piston machine 140, motor power electronics 130, control system150, and GTI 120. It will be understood that the present disclosure alsoapplies to powered systems that require a pulsed power profile, inaddition to generator systems that output a pulsed power profile. Forpurposes of discussion, the following descriptions are framed primarilyin terms of generators but will be applicable to powered systems aswell.

Free-piston machine 140 may include a system similar to that shown inFIG. 7, for example. In general, free-piston machine 140 may includetranslating assemblies (i.e., “translators”) that may undergoreciprocating motion relative to a stator under the combined effects ofgas pressures and electromagnetic forces. The translators may, but neednot, include permanent magnets that may generate a back electromotiveforce (emf) in phases of the respective stator. Note that, as usedherein, and as widely understood, back emf refers to a voltage inducedacross the stator windings from the moving translator. Motor powerelectronics 130 is configured to provide current to the phases of thestators. For example, motor power electronics 130 may expose phase leadsof phases of the stator to one or more buses of a DC bus, a neutral, aground, or a combination thereof.

Motor power electronics 130 may include, for example, insulated gatebipolar transistors (IGBTs), diodes, current sensors, voltage sensors,circuitry for managing Pulse Width Modulation (PWM) signals, any othersuitable components, or any suitable combination thereof. In someembodiments, motor power electronics 130 may interface with free-pistonmachine 140 via phase leads 135 which couple to phase windings of thestators, and motor power electronics 130 may interface with GTI 120 viaDC bus 125 (e.g., a pair of buses, one bus at a higher voltage relativeto the other bus). Bus 122 and bus 124 together form DC bus 125 insystem 100. For example, bus 122 may be at nominally 800 V relative to 0V of bus 124 (e.g., bus 122 is the “high” and bus 124 is the “low”). Bus122 and bus 124 may be at any suitable, nominal voltage, which mayfluctuate in time about a mean value, in accordance with the presentdisclosure. Accordingly, the term “DC bus” as used herein shall refer toa pair of buses having an established mean voltage difference, althoughthe instantaneous voltage may fluctuate, vary, exhibit noise, orotherwise be non-constant.

GTI 120 may be configured to manage electrical interactions between ACgrid 121 (e.g., three-phase 480 VAC) and DC bus 125. In someembodiments, GTI 120 is configured to provide electrical power to ACgrid 121 from free-piston machine 140 (e.g., a free-piston engine) viamotor power electronics 130. In some embodiments, GTI 120 may beconfigured to source electrical power from AC grid 121 to input tofree-piston machine 140 (e.g., a free-piston air compressor) via motorpower electronics 130. In some embodiments, GTI 120 manages electricalpower in both directions (i.e., bi-directionally to and from AC grid121). In some embodiments, GTI 120 rectifies AC power from AC grid 121to supply electrical power over DC bus 125. In some embodiments, GTI 120converts DC power from DC bus 125 to AC power for injecting into AC grid121. In some embodiments, GTI 120 generates AC waveforms of current andvoltage that are suitable for AC grid 121. For example, GTI 120 maymanage a power factor, voltage, and frequency of AC power injected intoAC grid 121.

Although shown as being coupled to AC grid 121 in FIG. 1, GTI 120 may becoupled directly to a load, a power source, another generator system,another GTI, any other suitable electric power system, or anycombination thereof. For example, generator system 100 may be in“islanding” mode or “stand-alone” mode, wherein AC grid 121 may be alocal AC grid, having an AC load.

While the present disclosure is described herein in terms of a GTI, itwill be understood that any suitable type of power inverter (alsoreferred to as an inverter) may be used. Moreover, in some embodiments,neither a GTI nor any other type of inverter need be used. For example,the power output of the DC bus may be coupled to a DC grid, a DC load,any other suitable DC structure or architecture, or any combinationthereof. Power fluctuations can affect many such systems. The use of aGTI is provided merely for purposes of illustration and not by way oflimitation.

FIG. 2 shows plot 200 of illustrative power and voltage time traces, inaccordance with some embodiments of the present disclosure. Power timetrace 210 and voltage time trace 220 may be representative of electricalconditions on DC bus 125 of FIG. 1. The horizontal axis of plot 200 ispresented in units representative of time (e.g., seconds, milliseconds,or number of sample time steps), while the left vertical axis ispresented in units representative of power (e.g., kW, horsepower) andthe right vertical axis is presented in units representative of voltage(e.g., volts). As illustrated in FIG. 1, illustrative power time trace210 is representative of power generation (e.g., from a free-pistonlinear generator), and accordingly includes primarily positive values.It will be understood that for motored conditions, which include powerbeing input rather than generated, the value of power time trace 210 isprimarily negative.

Power time trace 210 exhibits pulsed, or highly fluctuating, behavior,which is termed herein as a “pulsed power profile.” In some embodiments,power is extracted from, or supplied to, a free-piston machine duringboth expansion and compression strokes (e.g., strokes of an enginecycle, or an air compression cycle). For example, the power isproportional to velocity (e.g., power may be equal to the dot product offorce and velocity). Near the apices, under some circumstances, theelectric motors' ability to extract energy is reduced (e.g., due toforce limitations) as the translator speed reduces to 0 (e.g., near aturnaround position). For example, when the translator velocity is at ornear zero, it may be impractical to extract or supply power. Thereciprocating character of the free-piston machine (e.g., a linearfree-piston machine) thus may cause a pulsed power profile to beoutputted (e.g., when the free-piston machine is generating poweroutput), or a pulsed power profile to be required (e.g., when motoringthe free-piston machine with a power input).

For example, peaks 211, 213, 215, and 217 of power time trace 210 arecaused by the translator moving a first direction (e.g., eitherexpansion or compression), while peaks 212, 214, and 216 are caused bythe translator moving in a direction opposite to the first (e.g., eithercompression or expansion). When the translator changes its direction ofmotion (e.g., in between peaks, when translator velocities are at ornear zero), the power output is near zero. Voltage time trace 212exhibits relatively lower fluctuations, characteristic of, for example,the ability of a GTI to regulate a DC bus. Under some circumstances, apower utility grid may be unlikely to directly accept a pulsed powerprofile (e.g., having a large coefficient of variance) like power timetrace 210. Further, sizing a GTI to manage a pulsed power profile maypresent challenges. For example, at or near the peaks in power, theinstantaneous power may be much greater than the average power.Accordingly, an inverter may need to be sized to manage the peak powereven if the average power is significantly less.

In some embodiments, multiphase electromagnetic machines rely on a DCbus to provide, or remove, electrical energy. In some embodiments,multiphase electromagnetic machines include windings (e.g., windings ofcopper wire around ferrous cores), through which a controller currentflows, producing an electromagnetic force on a translating assembly(e.g., a piston assembly having permanent magnets). The windings may begrouped into phases (e.g., coupled in series), for which the current isindependently controllable. For example, the control of current inwindings of one or more phases of a linear, free-piston machine may relyon a DC bus to provide current to the windings via motor powerelectronics 130. In some circumstances, a DC bus is generated andmaintained by rectifying and regulating electrical power from an ACsource such as an AC grid. Due to the pulsed, non-steady character ofpower to or from free-piston machines, maintaining the DC bus likelybenefits from the use of GTIs, DC-DC converters, energy storage devices,and other components. The use of an energy storage device allows forfluctuations in electrical power to be smoothed, by the storage ofelectrical energy. The use of a DC-DC converter allows for fluctuationsin voltage (e.g., caused by the energy storage device sourcing orsinking electrical energy) to be smoothed.

In some embodiments, one or more GTIs are used. For example, if morethan one GTI is used, a DC bus may be divided into several DC buses(e.g., for redundancy). Motor power electronics (MPE) may be implementedin various ways. In some embodiments, an energy storage device (e.g., acapacitor bank, a battery bank) is divided, distributed, and integrateddirectly in one or more MPE modules.

In some embodiments, the voltage at an energy storage device undergoesfluctuations over time, because of energy storage and release processes.For example, in order to reduce cost and/or size and/or weight, it is ofinterest to limit the total capacitance of a capacitor bank on a DC bus,which has a direct consequence of increasing a voltage swing on the DCbus. In an illustrative embodiment, one or more capacitor banks may beconnected directly to one or more DC buses. Accordingly, the DC busvoltage varies over time. If the capacitor bank were sized with aminimum capacitance, then the voltage fluctuation would be significant.The voltage fluctuation causes, for example, overstress of the GTI andMPE, reduced efficiency of the GTI and MPE, or other ill effects. Insome circumstances, these detrimental effects may become significant,which requires an over-sizing of the capacitor bank.

For example, if an MPE circuit topology such as either 1) a half-bridgeper phase in a wye (or star) configuration, or 2) a half-bridge orfull-bridge per phase in an independent phase current (IPC)configuration, then the maximum phase voltage may be limited.Accordingly, the motor force may be limited. Phase windings may beadapted to reduce back emf, which may increase current, and associatedlosses in both a motor and corresponding MPE.

In some embodiments, an energy storage device is coupled to a DC bus,and a corresponding MPE, GTI, or both, include voltage step-upcapability.

FIGS. 3-6 show illustrative systems including a DC bus in accordancewith the present disclosure. The number of linear electromagneticmachines (LEMs) included in any of the systems is illustrative and mayrange from a single LEM to many LEMs. Any suitable LEM, or combinationthereof, may receive or provide a pulsed power profile, for which asmoothed power profile may be desired for coupling to an AC grid via aGTI. It will also be understood that although a GTI is shown in each ofthe systems of FIGS. 3-6, a GTI may be omitted, more than one GTI may beincluded, or a combination of a GTI and other suitable equipment may beused. It will also be understood that illustrative LEMs shown in FIGS.3-6 may correspond to systems (e.g., free-piston machines) that outputpower (e.g., engines), or require power (e.g., compressors). It willalso be understood that two or more components may be coupled together,yet still have one more components coupled in between. Accordingly, theterm “coupled” as used herein allows for a direct connection or aninteraction via an intervening component.

FIG. 3 shows a diagram of illustrative system 300, including energystorage device 330, in accordance with some embodiments of the presentdisclosure. In some embodiments, an energy storage device, or energystorage system, is used to compensate for the coefficient of variance inthe power output of a free-piston machine.

System 300 includes LEMs 350, 352, and 354, representing themotor-generators of one or more free-piston machines. Modules 340, 342,and 344 are configured to control phase currents in phases of respectiveLEMs 350, 352, and 354. As shown in FIG. 3, modules 340, 342, and 344are coupled to DC bus 335, which includes buses 322 (e.g., a “high” bus)and 324 (e.g., a “low” bus). Although shown as having “N” modules,coupled to respective LEMs, coupled to DC bus 335, in some embodiments,a single module may be coupled to DC bus 335. DC bus 325 is coupled toGTI 320 (e.g., via suitable terminals), and also to DC bus 335 (e.g.,via energy storage device 330). In some embodiments, the voltages of bus322 and bus 332 are substantially similar to each other, and thevoltages of bus 324 and bus 334 are substantially similar to each other.

In some embodiments, the power transmitted at any instant via DC bus 325and DC bus 335 differ, due to energy storage, and changes of energystored therein, in energy storage device 330. For example, a pulsedpower output from modules 340, 342, and 344 onto DC bus 335 may exhibitrelatively less fluctuation on DC bus 325 due to energy storage ofenergy storage device 330. In a further example, a pulsed powerrequirement for modules 340, 342, and 344 from DC bus 335 may sourcepower exhibiting less fluctuation from DC bus 325 due to energy storageof energy storage device 330. Accordingly, the presence of energystorage device 330 may reduce fluctuations of power, over time, on atleast one DC bus.

In some circumstances, the presence of energy storage device 330 ingenerator system 300 may cause voltage fluctuations, as energy is storedand released. In some embodiments, GTI 320 and modules 340, 342, and 344may accommodate the voltage fluctuations. For example, in the presenceof voltage fluctuations, modules 340, 342, and 344 may still be able tocontrol respective LEMs 350, 352, and 354. In a further example, in thepresence of voltage fluctuations, GTI 320 may still be able to controlrespective LEMs 350, 352, and 354.

In some circumstances, for example, GTI 320 and modules 340, 342, and344 may have difficulty accommodating the voltage fluctuations which maybe caused by energy storage device 330. Accordingly, a DC-DC convertermay be used to aid in maintaining lower voltage fluctuations on a DCbus.

A module (e.g., any or all of modules 340, 342, and 344), as referred toherein in the context of FIGS. 3-6, may include motor power electronics,control circuitry, sensors, phase leads and corresponding hardware, anyother suitable components or systems for managing current in phases ofan LEM, or any suitable combination thereof.

FIG. 4 shows a diagram of illustrative system 400, including energystorage device 430 and DC-DC converter 480, in accordance with someembodiments of the present disclosure. System 400 includes LEMs 450,452, and 454 (e.g., which may be included in one or more multiphaseelectromagnetic machines). Modules 440, 442, and 444 are configured tocontrol phase currents in phases of respective LEMs 450, 452, and 454.In some embodiments, modules 440, 442, and 444 may include powerelectronics, control circuitry, sensors, any other suitable components,or any suitable combination thereof. As shown in FIG. 4, modules 440,442, and 444 may be coupled to DC bus 425. GTI 420 couples DC bus 425 toAC grid 421. LEMs 450, 452, and 454 may each provide, or require, apulsed power profile on DC bus 425.

As shown in FIG. 4, DC-DC converter 480 couples energy storage device430 to DC bus 425. Accordingly, power transfer though DC-DC converter480 allows the power profile at DC bus 425 to be relatively smoother.For example, the energy stored in energy storage device 430 may exhibitlarge variations in time, effectively sourcing and sinking power to DCbus 425 from DC bus 435. DC-DC converter 480 may regulate the busvoltage of DC bus 425 to a constant setpoint (e.g., although it mayexhibit some fluctuation, perturbation, noise, or a combinationthereof). Plots 490 and 495 exhibit illustrative time traces, asdiscussed below.

Plot 490 shows illustrative voltage and power time traces representativeof DC bus 425. Both voltage and power exhibit relatively smallfluctuations, and accordingly, may be suitable for interacting with ACgrid 421 via GTI 420. Plot 495 shows illustrative voltage and power timetraces representative of DC bus 435. Both voltage and power exhibitfluctuations, with power swinging from positive to negative values.Accordingly, the use of DC-DC converter 480 and energy storage device430 may effectively isolate large fluctuations on DC bus 435 rather thanDC bus 425 to aid in GTI 420 interacting with AC grid 421.

In some embodiments, energy storage device 430 may include a capacitor,a capacitor bank, a battery, any other suitable device that storesenergy, or any combination thereof. In some embodiments, a suitableenergy storage device is capable of storing and releasing energy inquantities, and on time scales and at voltages, relevant to system 400.For example, energy storage device 430 may include a capacitor bank,coupled to DC bus 435 which may operate at a voltage approximately equalto, less than, or greater than, that of DC bus 425. In some embodiments,DC-DC converter 480 is bi-directional, and accordingly may output energyto DC bus 435, DC bus 425, or both (e.g., alternately in time) dependingon the temporal energy flows and storage of system 400.

In some embodiments, DC-DC converter 480 is configured to communicatewith a control system (e.g., similar to control system 150 of FIG. 1).For example, a control system may specify a DC voltage level of a DC-DCconverter. In a further example, a control system may receive statusinformation from a DC-DC converter (e.g., “running,” “faulted,” or“error”). In a further example, a control system may determine ameasurement of an output, an input, or a state of a DC-DC converter(e.g., a DC bus voltage, an amount of power transmitted, a current, avariance in a measurement, a temperature of the DC-DC converter, or anenergy loss).

For purposes of clarity, the following relationships will be definedherein.

Instantaneous power:

${Power} = {\frac{dE}{dt} = {{V(t)}*{I(t)}}}$in which V(t) is the voltage across suitable terminals, I(t) is thecurrent in the suitable terminals, and Power is the instantaneous changein energy with time at the suitable terminals.

Energy:

Energy = E(t) = E 0 + ∫₀^(t)Power * dtin which E(t) is the instantaneous energy at time t, E0 is a referenceenergy value (e.g., at time=0), Power is the instantaneous power, and dtis the differential time.

Energy in ESD:

${{Stored}\mspace{14mu}{Energy}} = {{E(t)} = {\frac{1}{2}{{CV}(t)}^{2}}}$in which E(t) is the instantaneous energy at time t, C is an effectivecapacitance (i.e., constant as shown, although the capacitance need notbe constant in time), and V(t) is the voltage across suitable terminalsof the ESD.

FIG. 5 shows a diagram of illustrative system 500, including energystorage device 530 and DC-DC converters 582 and 584, in accordance withsome embodiments of the present disclosure. System 500 includes LEMs550, 552, and 554 (e.g., which may be included in one or more multiphaseelectromagnetic machines). Modules 540, 542, and 544 are configured tocontrol phase currents in phases of respective LEMs 550, 552, and 554.In some embodiments, modules 540, 542, and 544 include powerelectronics, control circuitry, sensors, any other suitable components,or any suitable combination thereof. As shown in FIG. 5, modules 540,542, and 544 are coupled to DC bus 545. GTI 520 couples DC bus 525 to ACgrid 521. LEMs 550, 552, and 554 each provide, or require, a pulsedpower profile on DC bus 545.

System 500 includes three DC buses: DC bus 525, DC bus 585, and DC bus545. DC bus 525 is coupled to GTI 520 and to DC-DC converter 582. DC bus585 is coupled to DC-DC converter 582, DC-DC converter 584, and toenergy storage device 530. DC bus 545 is coupled to DC-DC converter 584and to modules 540, 542, and 544. In some embodiments, DC-DC converters582 and 584 are configured to communicate with a control system (e.g.,similar to control system 150 of FIG. 1).

Plots 590, 593, 596, and 599 show illustrative voltage and powerprofiles along the DC buses. Plot 599 shows representative voltage andpower profiles provided by, or required to, modules 540, 542, and 544.The pulsed power profiles characteristic of LEMs 550, 552, and 554 causethe power to fluctuate on DC bus 545. DC-DC converter 584 is configuredto transmit power between DC bus 545 and DC bus 585. As shown in plots599 and 596, the pulsed power profile from LEMs 550, 552, and 554 isalso present on DC bus 585. Energy storage device 530 is configured tostore and release electrical energy on DC bus 585 such that the powerprofile on DC bus 585 at DC-DC converter 582 exhibits relatively lessfluctuation than the power profile on DC bus 585 at DC-DC converter 584,as shown by plots 593 and 596. While the voltage across DC bus 585 maybe approximately the same everywhere between DC-DC converters 582 and584, the power profiles on DC bus 585 at DC-DC converters 582 and 584may differ from each other due to energy accumulation in energy storagedevice 530. The relatively smoothed power profile on DC bus 585 at DC-DCconverter 582, shown in plot 593, is transmitted by DC-DC converter 582to DC bus 525. Accordingly, the fluctuation in power on DC bus 525 isreduced compared to the fluctuation in power on DC bus 545, andaccordingly may be appropriate for GTI 520.

FIG. 6 shows a diagram of illustrative system 600, including energystorage device 630 and integrated DC-DC converters as part of GTI 620and module 642, in accordance with some embodiments of the presentdisclosure. System 600 includes LEM 650 which may be included in one ormore multiphase electromagnetic machines. Any suitable number of LEMsmay be included in system 600 coupled to DC bus 625, although only asingle LEM is illustrated in FIG. 6. Module 640 is configured to controlphase currents in phases of LEM 650. In some embodiments, modules 640includes power electronics, control circuitry, sensors, any othersuitable components, or any suitable combination thereof. As shown inFIG. 6, module 640 is coupled to DC bus 625. GTI 620 couples DC bus 625to AC grid 621. LEM 650 may provide, or require, a pulsed power profileon DC bus 625.

System 600 includes DC bus 625 coupled to GTI 620 and to energy storagedevice 630. GTI 620 may include a DC-DC converter having any suitabletopology, and module 640 may include a DC-DC converter having anysuitable topology. Accordingly, in some embodiments, DC bus 625 has acorresponding voltage that is designed for, optimized for, or otherwiseappropriate for, energy storage device 630. In some embodiments, GTI 620and module 640 include step-up converters, so that DC bus 625 operatesat a voltage larger than that used to cause current flow in windings ofLEM 650.

While some inverters are Buck-derived and thus cannot step up voltage,GTI 620, module 640, or both, may include suitable topologies that maystep up voltage (e.g. buck-boost converter, or Ćuk converters). In someembodiments, the use of a GTI, module, or both having DC-DC conversionfunctionality reduces cost, improves efficiency, simplifies the system,or results in a combination thereof.

Plots 690 and 695 show representative voltage and power profiles alongDC bus 625. Plot 599 shows representative voltage and power profilesprovided by, or required as an input to, module 640. The pulsed powerprofile characteristic of LEM 650 causes the power to fluctuate on DCbus 625 at module 640. Energy storage device 630 is configured to storeand release electrical energy on DC bus 625 such that the power profileon DC bus 625 at GTI 620 exhibits relatively less fluctuation than thepower profile on DC bus 625 at module 640, as shown by plots 690 and695. While the voltage on DC bus 625 may be approximately the same alongthe bus lines, the power profiles on DC bus 625 at GTI 620 and module640 may differ from each other due to energy accumulation in energystorage device 630. The relatively smoothed power profile on DC bus 625at GTI 620, shown in plot 690, may be transmitted to AC grid 621.Accordingly, the fluctuation in power on DC bus 625 is reduced at GTI620 compared to the fluctuation in power on DC bus 625 at module 640,and accordingly may be appropriate for GTI 620.

In some embodiments, GTI 620, module 640, or both, communicate with acontrol system (e.g., similar to control system 150 of FIG. 1). Forexample, a control system may specify a DC voltage level of a DC-DCconverter. In a further example, a control system may receive statusinformation from an integrated DC-DC converter (e.g., “running,”“faulted,” or “error”). In a further example, a control system maydetermine a measurement of an output, an input, or a state of anintegrated DC-DC converter (e.g., a DC bus voltage, an amount of powertransmitted, a current, a variance in a measurement, a temperature of acomponent, or an energy loss in a component).

FIG. 7 shows a cross-section view of an illustrative free-piston machine700, in accordance with some embodiments of the present disclosure.Free-piston assemblies 710 and 720 include respective pistons 712 and752, respective pistons 782 and 787, and respective translator sections751 and 756. Free-piston machine 700 includes cylinder 730, having bore732, which may house a high-pressure section (e.g., a combustionsection) between pistons 712 and 752.

In some embodiments, free-piston machine 700 includes gas springs 780and 785, which may be used to store and release energy during a cycle inthe form of compressed gas (e.g., a driver section). For example,free-piston assemblies 710 and 720 may each include respective pistons782 and 787 in contact with respective gas regions 783 and 788 (e.g.,high-pressure regions).

Cylinder 730 may include bore 732, centered about axis 770. In someembodiments, free-piston assemblies 710 and 720 may translate along axis770, within bore 732, allowing the gas region in contact with pistons712 and 752 to compress and expand.

In some embodiments, free-piston assemblies 710 and 720 includerespective magnet sections 751 and 756, which interact with respectivestators 752 and 757 to form respective linear electromagnetic machines750 and 755. For example, as free-piston assembly 710 translates alongaxis 770 (e.g., during a stroke of an engine cycle), magnet section 751may induce current in windings of stator 752. Further, current may besupplied to respective phase windings of stator 752 to generate anelectromagnetic force on free-piston assembly 710 (e.g., to affectmotion of free-piston assembly 710).

It will be understood that the present disclosure is not limited to theembodiments described herein and can be implemented in the context ofany suitable system. In some suitable embodiments, the presentdisclosure is applicable to reciprocating engines and compressors. Insome embodiments, the present disclosure is applicable to free-pistonengines and compressors. In some embodiments, the present disclosure isapplicable to combustion and reaction devices such as a reciprocatingengine and a free-piston engine. In some embodiments, the presentdisclosure is applicable to non-combustion and non-reaction devices suchas reciprocating compressors and free-piston compressors. In someembodiments, the present disclosure is applicable to gas springs. Insome embodiments, the present disclosure is applicable to oil-freereciprocating and free-piston engines and compressors. In someembodiments, the present disclosure is applicable to oil-freefree-piston engines with internal or external combustion or reactions.In some embodiments, the present disclosure is applicable to oil-freefree-piston engines that operate with compression ignition, sparkignition, or both. In some embodiments, the present disclosure isapplicable to oil-free free-piston engines that operate with gaseousfuels, liquid fuels, or both. In some embodiments, the presentdisclosure is applicable to linear free-piston engines. In someembodiments, the present disclosure is applicable to engines that can becombustion engines with internal combustion/reaction or any type of heatengine with external heat addition (e.g., from a heat source or externalreaction such as combustion).

It will be further understood that, while the present disclosure isdescribed in the context of free-piston machines, the concepts disclosedherein are applicable to any other suitable non-steady systems. The useof free-piston machines herein is merely for purposes of brevity andclarity.

The foregoing is merely illustrative of the principles of thisdisclosure and various modifications may be made by those skilled in theart without departing from the scope of this disclosure. The abovedescribed embodiments are presented for purposes of illustration and notof limitation. The present disclosure also can take many forms otherthan those explicitly described herein. Accordingly, it is emphasizedthat this disclosure is not limited to the explicitly disclosed methods,systems, and apparatuses, but is intended to include variations to andmodifications thereof, which are within the spirit of the followingclaims.

What is claimed is:
 1. A power management system that receives a poweroutput from a free-piston power generator comprising: a first DC buscontrolled by an inverter; a first DC-DC converter coupled to the firstDC bus and configured to provide a second DC bus, wherein the firstDC-DC converter is configured to output a smoothed power output to theinverter relative to the power output; at least one energy storagedevice coupled to the second DC bus; a second DC-DC converter coupled tothe second DC bus and to the free-piston power generator, wherein thesecond DC-DC converter is configured to receive the power output fromthe free-piston power generator.
 2. The power management system of claim1, wherein the second DC-DC converter is coupled to the free-pistonpower generator by a third DC bus, further comprising circuitry coupledto the third DC bus and configured to: control a plurality of currentsin a plurality of respective phases of the free-piston power generatorto generate the power output.
 3. The power management system of claim 1,wherein the second DC bus comprises: a first bus at a first voltage; anda second bus at a second voltage lower than the first voltage.
 4. Thepower management system of claim 1, wherein the inverter is coupled toat least one of a power grid and a load.
 5. The power management systemof claim 1, wherein the at least one energy storage device comprises atleast one capacitor.
 6. The power management system of claim 1, whereinthe at least one energy storage device is configured to store andrelease electrical energy from the second DC bus.
 7. The powermanagement system of claim 1, wherein the smoothed power output has acorresponding voltage, and wherein the first DC-DC converter isconfigured to regulate the voltage.
 8. The power management system ofclaim 1, wherein the first DC-DC converter is adjustable, the powermanagement system further comprising a control system configured toadjust one or more operating characteristics of the first DC-DCconverter.
 9. The power management system of claim 1, wherein the firstDC-DC converter comprises a step-down converter.
 10. A power managementsystem that receives power from a power input and provides a pulsedpower output to a free-piston machine comprising: a first DC buscontrolled by an inverter coupled to the power input; a first DC-DCconverter coupled to the first DC bus and configured to provide a secondDC bus; at least one energy storage device coupled to the second DC bus;a second DC-DC converter coupled to the second DC bus, wherein thesecond DC-DC converter is configured to output the pulsed power outputto the free-piston machine relative to the power input.
 11. The powermanagement system of claim 10, wherein the second DC-DC converter iscoupled to the free-piston machine by a third DC bus, further comprisingcircuitry coupled to the third DC bus and configured to: control aplurality of currents in a plurality of respective phases of an electricmotor of the free-piston machine to provide the pulsed power output. 12.The power management system of claim 10, wherein the second DC buscomprises: a first bus at a first voltage; and a second bus at a secondvoltage lower than the first voltage.
 13. The power management system ofclaim 10, wherein the inverter is coupled to a power grid.
 14. The powermanagement system of claim 10, wherein the at least one energy storagedevice comprises at least one capacitor.
 15. The power management systemof claim 10, wherein the at least one energy storage device isconfigured to store and release electrical energy from the second DCbus.
 16. The power management system of claim 10, wherein the pulsedpower output has a corresponding voltage, and wherein the second DC-DCconverter is configured to regulate the voltage.
 17. The powermanagement system of claim 10, wherein the second DC-DC converter isadjustable, the power management system further comprising a controlsystem configured to adjust one or more operating characteristics of thesecond DC-DC converter.
 18. The power management system of claim 10,wherein the second DC-DC converter comprises a step-down converter.